Recently, sequential build-up boards have been developed in order to package various electronic elements with high density. The sequential build-up boards are made of a plurality of circuit printed boards placed one on another. The circuit printed board is produced by producing electroconductive via holes such as blind holes or through holes in a circuit board having on each surface side a predetermined circuit pattern.
The blind holes are produced by forming holes in a metal coat of a circuit printable board by etching and then forming holes in an insulating substrate of the circuit printable board by means of a punch, a drill, a carbon dioxide laser, a UV-YAG laser, or an excimer laser.
Japanese Patent Provisional Publication 10-154730 describes a process for producing a blind via hole which comprises the steps of patterning a copper layer of a two layer-CCL (Copper Clad Laminate employing a polyimide film) utilizing a photoresist coat placed on the copper layer; removing the polyimide film in the patterned area by CO2 laser to form a blind hole; desmearing a polyimide resin deposited in the bottom of the blind hole; removing a portion of the copper layer and a small amount of polyimide resin by means of etching and desmearing; making the blind hole electroconductive by copper-plating to convert a blind via hole.
In the production of via holes, metal flashes are generally generated. The metal flashes are then removed by buffing or dry-blasting. The desmear (clearing the inside of the via hole) is performed by a wet desmear method using an aqueous alkaline manganese salt solution.
The buffing inadvertently causes elongation in the machine direction and then brings about anisotropic dimensional variation of the metal/polyimide film composite film. The polyimide wall in the via hole should be cleaned by an additional desmear procedure. The dry blasting may produce dusts in the blasting procedure. Therefore, the dry blasting is not advantageously employed in a clean room. Moreover, abrasive grains attached to the metal/polyimide film composite in the course of the blasting procedure disturbs adhesion of a photoresist in the following photolithographic procedure. The wet desmear is carried out under heating in a strong alkaline solution. Therefore, the polyimide film is likely damaged and cracks may be produced in the polyimide film.